The Aquafeins were sea creatures which lived in the oceans. They were blue fish-like humanoids who had fins and long tendrils. They were able to live outside the water too as demonstrated when they were in an undersea air pocket. They were relatively resistant to ray guns, as a blast from wouldn't kill one but it would hurt it substantially.
The First Doctor, Ian and Barbara encountered an Aquafein when they landed in the ocean. It lifted Barbara up. In order to save her, Ian fired a ray gun at it to which it let her go. However more turned up. (COMIC: Escape from the Aquafien)
Behind the scenes
- Despite the story being called Escape from the Aquafien, the species are called Aquefeins.